The present invention relates generally to communications, and more specifically to a novel and improved method and apparatus for frequency tracking using inner and outer loops.
Wireless communication systems are widely deployed to provide various types of communication such as voice and data. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other modulation techniques. A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the xe2x80x9cTIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (the IS-95 standard), (2) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Project 2xe2x80x9d (3GPP2) and embodied in a set of documents including xe2x80x9cC.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d the xe2x80x9cC.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,xe2x80x9d and the xe2x80x9cC.S0024 cdma2000 High Rate Packet Data Air Interface Specificationxe2x80x9d (the cdma2000 standard), and (4) some other standards. Non-CDMA systems include the AMPS and GSM systems.
Wireless communication systems commonly utilize local frequency references within various communication devices, such as base stations and mobile stations. The frequency references are used to modulate and demodulate transmitted data. For improved communication performance, and to maximize system resources, it is desirable for the various frequency references to be synchronized. However, inherent frequency differences between any two frequency references commonly exist due to variations in the manufacturing processes, among other factors. In addition, mobility of communication devices within a system (or mobility of obstacles off which reflected signals are received) can introduce frequency deviations due to Doppler. Frequency tracking loops are deployed to track and mitigate frequency errors between two communicating devices. Receiving stations may deploy inner tracking loops to track various multipath or soft-handoff signal components being received. In addition, an outer tracking loop may be deployed to track frequency differences in the local frequency reference of the receiving station. The interplay between the inner and outer loops can cause instability, leading to ineffective frequency tracking and thus to deterioration in communication performance. In addition, the response time, or convergence, of the various tracking loops can have an impact on performance as well. There is therefore a need in the art for inner/outer loop tracking that is stable and provides desirable loop convergence characteristics.
Embodiments disclosed herein address the need in the art for inner/outer loop tracking that is stable and provides desirable loop convergence characteristics. In one aspect, the contribution from any one inner loop to the tracking function of the outer loop is limited, to prohibit any one received signal component from dominating the outer loop. In another aspect, the rate of outer loop tracking variation is controlled to provide inner and outer loop stability. Various other aspects are also presented. These aspects have the benefit of providing stable inner and outer loop control, as well as efficient convergence and tracking by the various loops, resulting in reduced frequency error and improved communication performance.
The invention provides methods and system elements that implement various aspects, embodiments, and features of the invention, as described in further detail below.